As a cushion for a seat back of a seat, which is formed by causing a foam resin padding and a three-dimensional cover covering the foam resin padding to be integrated with each other by a die apparatus, there is proposed a cushion for a seat back, which is disclosed in Japanese Patent Application Laid-Open No. 2005-262628, for example.
When the cushion is to be formed by the die apparatus, the three-dimensional cover is laid in the die apparatus. In this condition, expandable resin is poured into an interior of the three-dimensional cover through the die apparatus and allowed to foam, whereby a padding of the foaming resin integral with the cover is formed. In this way, a finished cushion is formed. After the finished cushion is formed and removed from the die apparatus, a seat back frame for supporting the cushion is incorporated in the padding. For this purpose, the die apparatus includes a core for forming a cavity opened at a rear side of the padding for receiving the seat back frame. When the three-dimensional cover is laid in the die apparatus, the core is inserted in the three-dimensional cover. In order that the three-dimensional cover can receive the core when it is laid in the die apparatus, a rear side section of the three-dimensional cover is adapted to be openable and closable with respect to the cover by slide fastener means.
Referring now to FIGS. 1 and 2, the related art cushion will be discussed hereinafter in order to facilitate understanding of the present invention.
Referring to FIG. 1, the cushion includes the three-dimensional cover 33, in an interior of which the foam resin padding (not shown) is integrated with the three-dimensional cover 33. The cover 33 includes a front side piece 33a, a rear side piece 33c spaced apart from the front side piece 33a and opposed to the front side piece 33a, a peripheral piece 33b disposed between the front side piece 33a and the rear side piece 33c, extending along peripheral edges of the front side and rear side pieces 33a, 33c except lower edges of the front side and rear side pieces 33a, 33c, and sewed to the peripheral edges of the front side and rear side pieces 33a, 33c at edges of the peripheral piece 33b, and a lower side piece 34 sewed at an edge thereof to a lower edge of the front side piece 33a. The rear side piece 33c is formed substantially into a U-shape and includes a pair of spaced apart side sections 33d, an upper section connected to upper edges of the spaced apart side sections 33d and interconnecting the spaced apart side sections 33d, and a belt-shaped intermediate piece 33e. The intermediate piece is attached at an upper edge thereof to a lower edge of the upper section of the rear side piece 33c and is adapted to be coupled at left and right side edges thereof to inner edges of the side sections 33d by first and second slide fastener means, whereby an opening of the rear side piece 33c is openably closed by the intermediate piece 33e. The first slide fastener means includes a pair of first and second zipper tapes 35a which are provided along the left side edge of the intermediate piece 33e and an inner edge of a left side section of the rear side piece 33c, and a first slider assembly (not shown). The first slider assembly, supported by the first and second zipper tapes, is included for zipping and unzipping the first zipper tape with respect to the second zipper tape. Moreover, the second slide fastener means includes a pair of third and fourth zipper tapes 35b which are provided along the right side edge of the intermediate piece 33e and an inner edge of a right side section of the rear side piece 33c, and a second slider assembly (not shown). The second slider assembly, supported by the third and fourth zipper tapes, is included for zipping and unzipping the third zipper tape with respect to the fourth zipper tape. Thus, the intermediate piece 33e is configured so as to be separatable from the side sections 33d of the rear side section 33c by the first and second slide fastener means, so that the cover is openable at the rear side section thereof. The foam resin padding in the cover 33 is integrated with the respective sections of the cover 33 except the intermediate piece 33e and the lower side piece 34.
The cushion which includes the cover 33 constructed as discussed above and the foam resin padding is formed by utilizing a die apparatus 3 which is partially illustrated in FIG. 2. The die apparatus 3 includes a first die means 30 for forming front and rear sections of the cushion and a peripheral section of the cushion, a core 31 for forming a cavity opened at a rear side of the foam padding 36 for receiving a seat back frame (not shown), and a second die means 32 which is adapted to be combined with the first die means 30 and clamped against the first die means 30, when the cushion is to be formed by the die apparatus. The first die means 30 is provided with a first sectional die means 30a for forming the rear side section of the cushion, and a second sectional die means 30b for forming the peripheral section of the cushion. The first sectional die means 30a is coupled to the second sectional die means 30b by a first hinge means 30c. The second sectional die means 30b is coupled to the first die means 30 by a second hinge means 30d. Thus, the first and second sectional die means 30a, 30b can be opened and closed with respect to the first die means 30 by the first and second hinge means 30c, 30d. 
When the cushion is to be formed by the die apparatus, the second die means 32 is removed from the first die means 30, the first and second sectional die means 30a, 30b are opened with respect to the first die means 30 by the first and second hinge means 30c, 30d, and the cover 33 is then laid on a die surface of the first die means 30 with the intermediate piece 33e being separated from the side sections 33d of the rear side piece 33c of the cover 33 by unzipping the first and third zipper tapes of the intermediate piece 33e from the second and fourth zipper tapes of the side sections 33d of the rear side piece 33c. Then, the core 31 is inserted within the three-dimensional cover 33 and the first and second sectional die means 30a, 30b are closed with respect to the first die means 30 by the hinge means 30c, 30d. At this time, each of the zipper tapes 35a, 35b provided at the both side sections 33d of the substantially U-shaped rear side piece 33c is located within a space X between the second die means 32 and the core 31, and each of the both side sections 33d of the substantially U-shaped rear side piece 33c becomes pinched by a protrusion 31a of the core 31 and the first sectional die means 30a. In this condition, the second die means 32 is combined with the first die means 30 and clamped against the first die means 30, and expandable resin is poured into the interior of the cover 33 and foams in the interior of the cover 33, whereby the cushion in which the padding 36 of the foaming resin is integrated with the cover 33 and formed with the cavity for receiving the seat back frame is formed. Each of the both side sections 33d of the substantially U-shaped rear side piece 33c is pinched by the protrusion 31a of the core 31 and the first sectional die means 30a as discussed above, so that the foaming resin will not adhere to a surface of the cover 33 and/or the zipper tapes 35a, 35b. 
However, in the related art cushion constructed as discussed above, the front side piece 33a is sewed to the peripheral piece 33b, and the peripheral piece 33b is sewed to the rear side piece 33c, so that there are many sewed portions between the front and rear side pieces 33a, 33c and the peripheral piece 33b, thus detracting from the visual appearance of the cushion.
Moreover, if it is necessary to provide a pocket on a rear surface of the related art cushion, the pocket is obliged to be provided on the belt-shaped intermediate piece from the viewpoint of facilitating the assembling of the cushion. Therefore, only a pocket having a narrow width can be provided on the intermediate piece but a pocket having a wide width can not be provided on the intermediate piece.